Allow for custom commit messages that include details from multiple commits

[D3SOX]

When pushing multiple commits at once the {msg} template only provides the message of the latest commit. Is it possible to have all commits' messages put into the MESSAGE parameter and therefore in the published commit message?

It would be great if this could also be applied to {sha}/{long-sha}.

I want the published commit message to be something like:

Update build with new commits
- feat: new feature
- fix: fix stuff
- ...
Commits: abcdef, fedcba, ...

[s0]

I'll have a think about the best way to do this... it's not obvious how to make this configurable, it may perhaps need to allow for multiple placeholders

The other complex element is that determining the last HEAD sha is non-trivial too, as there's no guarantee that the message is of a particular format (and that nothing else has been pushed), and there's no git metadata that will reffer to the git sha used in the upstream repo